Ripening processes and ripening rooms, broadly defined as including ripening rooms, chambers, or truck containers, are generally known and have been used for many years in the fruit industry for ripening fruit, such as bananas. One important working principle in ripening room construction is that air pressure and air flow should be uniform throughout the air flow circuit in the room, which typically extends from above boxes and pallets containing bananas, through air flow pathways that circulate through the banana boxes, and back to ventilating fans, in order to effect uniform ripening of all bananas in the room.
Conventional ripening rooms are usually of either a "chamber" or truck "container" variety and typically comprise an operational area and a cooling area. In conventional ripening chambers, perishable fruit, such as bananas, are stacked on pallets or in boxes in a configuration known as a "tight-stack", in which two or more rows of tightly stacked boxes separated by a corridor are spaced-apart from the side walls of the chamber, while being positioned in the operational space of a substantially enclosed ripening room having generally rectangular walls, floor, and ceiling. In truck containers, the boxes are generally stacked in what is known as a "pyramid stack," in which each box in each row is spaced-apart from the next box in that row and the rows are also spaced-apart from each other, such that spaces are provided about all sides of the boxes. The next level of boxes in a pyramid stack would not be placed directly over the boxes below it, but rather would be placed off-center, so that each box is partially stacked on top of two boxes below it, in typical pyramid fashion.
Boxes containing bananas typically have holes in them to encourage air circulation about the bananas. The ripening room is generally supplied with air which is cooled by a refrigeration system, in order to maintain a predetermined fruit temperature. The cooled air is circulated throughout the substantially enclosed room at discretely controlled temperatures for a fixed length of time, until the bananas ripen.
A typical ripening cycle would likely include a period of gassing in which ethylene gas, for example, is introduced into and circulated throughout the room at a fixed setpoint temperature, followed by a period of air circulation in which forced air (along with the ethylene mixture) is circulated throughout the room or container. Some of the ethylene gas may then be removed, followed by a relatively long period where air is circulated by the fan(s) while the setpoint temperature of the supplied air is lowered in discrete steps by a thermostatic controller. The thermostatic controllers for conventional ripening programs typically provide for distinct and sudden drops in the setpoint temperature at certain time intervals. For example, in a conventional four day ripening cycle, the air temperature setpoint temperature might be set to 18.degree. C. for two days, then instantaneously drop to 16.5.degree. C. for one day, and again instantaneously drop to 14.5.degree. C. for the final day.
Refrigeration systems have been used for many years of the type which use a compressor to provide a "refrigerant" also known as "coolant" flowing through a closed-loop system. The compressor increases both the pressure and the temperature of the vaporous refrigerant before the refrigerant is directed into a condenser. As it passes through the condenser, the vaporous refrigerant is cooled and condensed to a liquid, while releasing heat to the surrounding environment, usually with the aid of a fan. The liquid refrigerant is then directed to a thermal expansion valve which provides a somewhat controlled release of the high pressure liquid refrigerant into a series of coils, commonly called an evaporator. As it passes through the thermal expansion valve, the liquid refrigerant undergoes a change of state from a high pressure liquid to a lower pressure vapor, while extracting thermal energy from the atmosphere surrounding the evaporator. The vaporous refrigerant is then drawn into the compressor to close the loop and to restart the refrigeration process cycle.
The typical conventional ripening room cooling system operates in an on/off switching mode, in which cooling capacity variations are made by a solenoid valve, thereby shortening or extending the supply time of the cooling agent to the cooler. Changes in the cooling requirements and ambient temperature conditions throughout the ripening process affect the variable coolant evaporation temperature. However, conventional banana ripening rooms function based on a constant evaporation temperature, which may be set to a constant by a back pressure regulation valve in the suction line, calculated for the maximum capacity requirement of cooling during the peak of fruit respiration. As a result, conventional ripening room cooling control systems operate inefficiently.
The rear of the room (i.e., chamber or container) has a "cooling area," separated from the "operational area," which contains the refrigeration system that cools the air before it is supplied to the operational area of the room. Conventional cooling systems for ripening bananas generally consist of one or more air coolers with fans placed on a "supplemental" wall dividing the cooling area from the operational area, or under a roof in front of a supplemental wall toward the rear of the ripening room. The forced air fans are often placed behind the cooler in conventional ripening systems. In one example of a conventional ripening room, the supplemental wall closing off the cooling area from the operational area is a "reeled roller blind," which comprises a tarp that rolls on to and off of a rod much like a window shade, and which separates the chamber space into areas of high pressure and low pressure.
In conventional ripening rooms using a tight-stack configuration a typical air flow circuit path supplies air into an operational area over the top of the boxes (which may be partially or fully covered on top by a tarp), down along the spaces between the sides of the room and the banana boxes, through the holes of the sides of the boxed bananas, and into the corridor between the two rows of tightly stacked boxes. The air flow circuit then returns the air to a cooling area in the rear of the room behind the supplemental wall, where the air passes through a cooler/evaporator in the cooling area, through one or more fans between the cooling area and the operational area, and returns the air to the operational area where the bananas are stacked. This configuration is sometimes referred to as a "Del Monte" ripening room.
In conventional ripening containers using a pyramid stack configuration, the container is similarly divided into an operational area where the boxes are stacked, and a cooling area where the forced air is cooled. In this stacking configuration, some of the boxes typically are placed against the side walls of the operational area of the ripening container. A typical air flow circuit path in a pyramid stack configuration forms a single large loop starting at the cooler in the cooling area at the rear of the ripening container, to the front of the container, then back to the cooling area. More specifically, the fans supply air into the operational area (and ultimately to the front of the ripening container), in and around the top portions of boxes through the spaces between the boxes and the holes in the boxes, and the air returns to the cooling area via the bottom portions of the boxes. While most of the supplied air reaches the front of the ripening container, some of this air bypasses the route and is essentially short-circuited into the bottom portions of the boxes. As with the tight-stack configuration, all of the air returns to the cooling area, then passes through the cooler, through the fans and back into the operational area.
The conventional ripening processes and rooms described above have exhibited several deficiencies which are known in the art but which, until now, have not been resolved. First, the bananas do not ripen uniformly and much of the fruit undergoes considerable shrinkage and lessened fruit quality when placed in conventionally constructed ripening rooms, because the cooling air does not circulate uniformly through the fruit. This lack of uniform air flow results in uneven air pressure, uneven cooling, and hence uneven ripening of the fruit. Whether the cooled air initially is forced over the top of the fruit or under the bottom of the fruit, the same circulation problem exists, particularly in chamber-type ripening rooms, because a relatively open free space separates the boxes from the conventionally constructed walls and ceiling of the room. As a result, the air flow is not equally distributed through all of the boxes.
For example, in a conventional tight-stack configuration, rather than descend along the sides of the ripening room walls, most of the air supplied from the fan(s) in the rear of the ripening room takes the path of least resistance, and passes over the fruit until reaching the front vertical wall (at the "far" end of the room from the fans), at which point the cooled air descends upon the bananas stacked near this front wall with a pressure that is greater than in other portions of the supply air pathway. The air is then drawn through the banana boxes by the same fan(s) in the rear of the ripening room. Because the higher air pressure in the front of the room causes the fruit situated there to receive a greater quantity of cooled air, those bananas stay greener in color, while the bananas in the middle and rear of the room remain at warmer temperatures and ripen to a yellow color much more quickly. The difference in the air pressure between the front and back of the ripening room is particularly pronounced in short ripening cycles. Consequently, a person entering the front of the room to check the ripeness of the bananas by color sees the green bananas and is misled into believing that the container or room is filled with unripe bananas.
Another problem associated with conventional cooling systems used for banana ripening is that they have operated in an "on/off" mode, where the temperature is controlled merely by turning the refrigeration system either "full" on or "full" off, rather than adjusting the amount or temperature of cooled air introduced into the operational area to some intermediate point. In such conventional cooling systems, the output cooling capacity is controlled, for example, by switching a solenoid valve either completely on or off to regulate the supply line of the cooling agent into the air cooler. Changes in fruit temperature during the ripening cycle are made by varying the setpoint temperature of the air in relatively large, sudden discrete steps, under the control of a temperature controller or automatic thermostat. These stepwise ripening programs used in the prior art are flawed, in that they tend to cause dehydration and undercooling of fruit, particularly where temperature sensors are placed in the boxes among banana clusters at the air outlet so that the time delay in the sensing feedback loop is much longer than necessary. Suddenly changing the setpoint temperature, for example, from 18.5.degree. C. to 16.50.degree. C. causes the refrigeration system to work at full capacity and induces intensive cooling for long time periods, considerable dehydration, and lowering of fruit ripening quality.
The evaporating temperature of refrigeration systems in conventional ripening rooms varies because of the change in cooling requirements during the ripening process and the change of ambient temperature conditions in the enclosed space. However, conventional systems operate using thermostatic expansion valves based on a constant evaporation temperature, by using a back pressure regulation valve which adds to the problem of low relative air humidity and fruit dehydration.
Finally, yet another shortcoming of the conventional ripening room processes and designs is that the cooling systems run inefficiently, thereby expending excessive energy. As related above, the air circulation in the conventional rooms used for banana ripening purposes have installed air coolers with air fans at the rear of the ripening room on a wall, or under a roof in front of a wall. The bananas are generally packed in bags and the bags are put into the boxes. Both the bags and the boxes have holes to allow circulation throughout the boxes among the bags of bananas. The air is cooled in only one basic circuit, namely, drawn by fans from the boxes in the operational space, through the cooler in the cooling space, and back into the operational space to form a single loop. Consequently, in the conventional one-loop configuration, a refrigeration system with excessive cooling capacity is required to cool the entire volume of air throughout the cooling cycle.
There is a need in the art for an improved room construction and cooling system used in ripening perishable products such as bananas, in which air flow is uniformly distributed, setpoint temperature is accurately controlled and gradually varied to reduce energy expenditure, thereby improving the quality of the ripened products located throughout the ripening room. By using the principles of the present invention, the shortcomings of the prior art are overcome while uniformity of ripened bananas in the entire ripening room, lowering of banana shrinkage, and better banana quality is achieved.